1. Field of the Invention
The present invention relates to a self-focusing cavity having high electron capture efficiency in respect of moderate injection voltages. The invention is primarily applicable to the field of linear accelerators, especially of the type employed in the field of industrial control or in the medical field in which it contributes to an improvement in reliability and efficiency of use.
2. Description of the Prior Art
A linear accelerator essentially includes an electron gun for injecting charged particles (electrons in the majority of instances), to which is connected at the downstream end an accelerating structure provided with an array of cavities in aligned relation. The cavities are supplied by a microwave power signal. When the charged particles consist of electrons, for example, the electron gun is usually made up of a cathode placed opposite to a hole anode. Under the action of the electric field produced by the high voltage applied between the cathode and the anode, electrons are detached from the cathode. They increase in velocity before arriving at the anode and escape from the electron gun through the hole of the anode. The accelerating structure placed downstream of the electron gun then has to subject these electrons to a number of effects.
The main function of this structure is to capture as many injected electrons as possible. Another function is to group them together so that, when they pass through the accelerating cavities, they undergo homogeneous modifications of their kinetic energy. Another intended function is to focus the electrons in order to impart a greater quantity of motion to a bunch of grouped electrons at the moment of an impact on a target. And finally, the structure must impart sufficient acceleration to the electrons to endow them with the necessary energy.
In order to improve the focusing, it was first considered advisable to prevent defocusing of the beam of electrons to be accelerated, at the exit of the first cavity. In fact, at the exit of the first cavity, the electrons are not yet fully accelerated and their energies are below 1 Mev even if their velocities are already close to the velocity of light. As a result, the mass of these electrons is comparable to their mass when stationary. It is said that these electrons are in that case highly mobile within the beam.
In order to avoid the divergence to which mobile electrons are subjected, it has been proposed in a French patent application No. 83 14090 filed on Sept. 2nd, 1983 by the same Applicant, to increase the length of the first cavity. In fact, the successive cavities of an accelerator which resonate with respect to each other in a .pi. mode, for example, have microwave electromagnetic fields which are reversed in phase in successive cavities. Thus a bunch of grouped electrons encounters a strong electric field within a cavity at the moment when, within adjacent cavities, the electric field is strong but of opposite direction. The lengths of the cavities are such that said bunch of electrons passes out of a cavity at the moment at which the electric field becomes zero at all points. This bunch of electrons then enters the following cavity at the moment when a suitable phase of the microwave signal is set up within said cavity. If necessary, drift spaces may be placed between the cavities in order to promote this phase synchronism. The electron bunch is also accelerated within the following cavity aforesaid.
In the French patent application cited above, this synchronism is upset, at least at the exit of the first cavity. From a practical standpoint, the cavities are volumes in which microwave electromagnetic fields are such that an electric field is maintained in the vicinity of the cavity axes which are followed by the electrons as they pass through these cavities. Said cavities are each provided with an entrance hole for receiving the injected particles and with an opposite exit hole through which the particles are discharged. In accordance with conventional practice, the entrance and exit holes are provided with portions which project toward the interior of the cavity. These projecting portions which surround the holes are known as cavity noses. The cavity walls are natually of metal. The lines of the electric field in the vicinity of the holes cannot be at all points parallel to the axis of the cavity between its entrance hole and its exit hole. In fact, these field lines necessarily close on the cavity noses. If the electric field at the moment when the electron bunch leaves the cavity is still very strong and generally oriented toward the following cavity (if in the final analysis the condition of synchronism is complied with or rather if the electron energy gain is optimized), these field lines are then divergent in proximity to the exit hole. Since the electrons are highly mobile at this moment, they are highly subject to this divergence effect. The beam is thus defocused.
On the other hand, if this first cavity is increased in length while the amplitudes and frequencies of the electric field are maintained at identical values, the phase of said electric field will be reversed at the actual moment of passage of the electron bunch through the exit hole or prior to this moment. In consequence, the electric field lines which result from this reversal after cancellation are then generally oriented toward the entrance of the cavity at this instant. This has the effect of slowing-down the bunch of electrons, which is objectionable. But a further effect thereby achieved (by reason of the radial component oriented in the right direction of this reversed electric field) is a radial reconcentration of the emitted electrons. It is then necessary to adjust the increase in length of the first cavity in order to ensure that the gain in focusing is not too penalizing in regard to loss of kinetic energy. It has thus been possible to determine in this patent application that the length L.sub.1 of the first accelerating cavity of the linear accelerator must be such that: EQU L.sub.1 =k'.beta..lambda..sub.0.
In this formula, k' is a coefficient which is recommended as having to assume the value 0.5; .beta.is the ratio of the mean velocity of the electrons to the velocity of propagation of light and .lambda..sub.0 is the wavelength of the microwave produced.
The disadvantage of this technique appears in regard to capture efficiency. In fact, by reason of the slowing-down process thus produced, electrons injected by the electron gun (the last injected electrons of a bunch) arrive really too late with respect to the phase reversal of the electric field within the first cavity. In consequence, they are returned to the electron gun and are recovered by the anode. The efficiency of the accelerator therefore falls off. In order to obtain a sufficient output in spite of this difficulty, it is a tempting possibility to use an electron gun having a higher delivery. For example, the cathode of the emitting gun is heated to a higher temperature. This technique suffers from a major drawback related to the technology of cathodes. When employed under these conditions, electron guns do not have a long lifetime.
In a second French patent application No. 85 13416 filed on Sept. 10th, 1985 by the same Applicant, there was presented a device for pregrouping and acceleration of electrons in which the capture efficiency could be increased to a value in the vicinity of 100%. In its essential principle, this invention has made it possible to determine that, in order to increase the capture efficiency, it is only necessary to impose within the first cavity an electric field of sufficiently low strength to ensure that electrons coming from the electron gun are not prevented from reaching the exit of the first cavity. In practice, this entails the need to impose a value of said electric field which is lower than that imposed within the second cavity. In actual fact, the ratio of these electric fields is of the order of two. It may be stated that, at the moment of reversal of phase, the electric field at the entrance of the first cavity no longer prevents the electrons from passing through said cavity whilst the electric field at the exit of the first cavity acts as a potential barrier to be crossed by electrons which are in course of returning to the cathode (for example after reflection at the entrance of the second cavity having a more intense field). The height of the barrier is determined on the one hand by the phase of the microwave within the first cavity at the moment when the late electrons pass through the exit hole of said first cavity. This potential barrier is also intrinsically dependent in addition on the amplitude of the microwave itself. It is thus readily apparent that, by reducing the value of this amplitude, the height of said potential barrier is also reduced. It is this technique which forms the subject of the second patent application cited earlier.
Thus all the electrons pass through the potential barrier. In this case also, a compromise is determined in regard to the ratios of the amplitudes of the fields within the first and the second cavity. In the patent application in question, it is also stated that, preferably, the ratio of these amplitudes shall be within the range of 1.5 to 3 in the case of an electron gun which emits electrons with a high voltage of 80 Kv and in the case of a microwave in the S band resonating at 3 GHz. In accordance with the teachings of this patent application, it is necessary to satisfy the following relation: EQU E.sub.1 .multidot.L1.multidot.T.sub.1 .ltoreq.1.5 V.sub.0
In this formula, V.sub.0 is the energy of the electrons at the entrance of the first cavity (the numerical value V.sub.0 expressed in eV is equal to that of the injection high voltage expressed in V), E.sub.1 is the amplitude of the mean electric field within said cavity and T.sub.1 is a mean transit angle factor representing the ratio between the energy really acquired by the electrons and the energy which would be acquired by these latter if the synchronism between the electric field and the electrons were complied with at all points of the cavity. It is known that the mean transit angle factor is related to the phase shift .theta. of the electromagnetic wave between the entrance and the exit by the following relation: EQU T.sub.1 =sin (.theta./2)/(.theta./2).
In practice, with a mean phase shift between the entrance and the exit of the first cavity of the order of .pi. radiant, T.sub.1 has a value of approximately 0.64.
A combination of the two recommended solutions is possible in theory. It would consist in employing a first cavity of greater length than a first cavity which is normal for synchronization and in feeding this cavity so as to produce an electric field of lower strength than the field which prevails within the second cavity. From a practical standpoint, by modifying the size of the hole which provides a coupling between the first cavity and the second cavity, it is known to modify the coefficient of electromagnetic coupling between these two cavities so as to ensure that the electric field within the first cavity, which is induced by an electric field existing within the second cavity (this latter may in turn be induced by a field produced within a third cavity), is smaller than the field within the second cavity.
In practice, a combination of these two technical effects is possible only within a very high range of voltage of electron guns even in respect of a ratio of fields equal to two, for example. In the example described in the second patent application cited earlier, the electron gun voltage must have a minimum value of 80 kV. On the other hand, should it be found for particular reasons such as overall size (which finally permits a reduction in diameter of the beam) as well as cost that it is desirable to make use of electron guns having moderate voltage such as 40 kV, for example, the application of the formulae given above entails the need to ensure that the electric field to be maintained within the first cavity is limited to a low value. In one example, it has been possible to demonstrate that this electric field had to be limited to 4.5 MV/meter whereas the electric field within the second cavity was of the order of 20 MV/m.
In point of fact, when cavities are coupled with electric fields having differences of this order, the coupling is unstable. As a result, it is not possible to maintain a constant value of 4.5 MV/meter within the first cavity. The electric field fluctuates. The conditions of adjustment are difficult and unreliable.
Furthermore, the microwave electromagnetic energy stored within the first cavity is proportional to the square of the amplitude of the electric field and therefore decreases at a higher rate than this latter. The electrons of the first micro-bunches which arrive within the low-field cavity then pump the entire stored electromagnetic energy, with the result that the electrons of the last micro-bunches are no longer subjected to any field within the first volume. Under these conditions, the accelerator fails to operate.
The optimum performance which could in that case have been obtained by associating the two technical effects is no longer feasible.
The aim of the invention is to overcome these disadvantages by providing a self-focusing accelerating structure or in other words employing the self-focusing effect of the first patent application cited earlier with high electron capture efficiency while utilizing the technical effect of the second patent application cited but at moderate injection voltages. The essential idea of the invention consists in so adapting the shape of the first cavity that the electric field within said cavity is developed in a distinctly dissymmetrical manner with respect to the center of said cavity. This virtually consists in optimizing the shape of the field modulus after having determined its extension along the axis and the amplitude of its peak value.
To simplify, this first cavity has a first portion in which the field is of low strength followed at the downstream end by a second portion in which the field is of high strength. In the first portion, the electrons are moderately accelerated but are above all bunched. Since the late-arrival electrons are subjected to field phases which are generally more favorable than the electrons which arrived first, they catch up with these first electrons. At the moment of entry into the second portion of the first cavity, the electrons which are sufficiently grouped together are then accelerated over the greater part of this second portion of the first cavity. Then from the moment at which they are thus grouped together and accelerated, the electric field becomes zero and is finally reversed. Before they pass through the exit hole of said first cavity, they are then slowed-down to a slight extent (which is somewhat unfavorable) but are above all favorably self-focused by the radial component of the reversed electric field at the exit of the first cavity.
However, the dissymmetry of the field prevents any sharp deceleration and therefore any strong focusing force. Within the second cavity and possibly within a third cavity which constitute the remainder of the accelerator, the electrons then undergo known accelerating effects produced by these cavities. In addition, however, the position of the field rise within the second cavity is utilized in order to add a radially focusing effect. This was difficult in the prior art since the mean energy gain within the first cell was generally higher and resulted in more rigid bunches. The totality of the effects sought is accordingly obtained. Bunching takes place in the first portion of the first cavity, 100% capture is produced by the high accelerating field of the second portion of the first cavity which is exerted on a bunch being formed, and self-focusing is obtained by the absence of defocusing at the exit of the first cavity (and even slight focusing) combined with the intense focusing obtained at the entrance of the second cavity. It is noted that these three effects take place within two successive coupled cavities.